The present invention relates to telemetering systems and in particular to a telemetering system which measures a physical parameter of a body by sensing variations in the figure-of-merit of energy storage (Q) of the sensor circuit caused by variations in selected parameter.
Telemetering systems for detecting the temperature, blood pressure, or other physical parameter of a human or animal patient from a remote location are known. For example, a miniature adjustable telemetering device is disclosed in Pope, et al., U.S. Pat. No. 3,971,362 as including a thermistor in a sensor circuit to measure body temperature. The sensor circuit generates intermittent bursts of energy at its resonant frequency where the time interval between bursts in dependent upon the resistance of the thermistor so that measurement of the interval between pulses constitues a measurement of the temperature of the object. An external device is utilized to detect the burst of energy from the sensor circuit and measure the time interval between the bursts. Therefore, Pope, et al does not measure variations in the Q of the sensor circuit to obtain a temperature value of the object.
Other references disclosing somewhat similar devices to the Pope, et al. device include Fryer, U.S. Pat. No. 3,453,546, and Murate, U.S. Pat. No. 3,682,160. For example, in Fryer, the ratio between a constant time interval and a time interval which varies dependent on the temperature is measured as a means of measuring temperature. Murata uses a similar system to measure acidity. Neither of these systems rely on sensing variations in the Q of the sensor circuit to measure the physical parameter, however.
A system utilizing a passive sensor device which is rung at the resonant frequency by an oscillating transmitted signal is disclosed in Honig, U.S. Pat. No. 3,218,638. However, in that system, the sensor circuit requires a pair of resonant tank circuits, one having a fixed resonant frequency and the other having a resonant frequency which varies in response to the resistance of a thermistor. A temperature measurement is obtained by comparing the fixed resonant frequency against the variable resonant frequency. Honig therefore does not measure variations in the Q of the sensor circuit.
Another patent which discloses the concept of sensing the ringing of a tuned sensor circuit is Nagumo, et al., U.S. Pat. No. 3,229,684. The sensor circuit in Nagumo stores energy from the transmitted signal and then reradiates that energy only after a period of time has elapsed after the termination of the transmitted energizing signal burst. The time period between the termination of the transmitted signal and initial reradiation of the signal is dependent on the value of a circuit component which is parameter sensitive so that the delay time constitutes a measurement of the parameter. Therefore, Nagumo likewise does not measure sensor circuit Q to indicate the value of the parameter.
The above-identified devices basically fall into one the two categories. In the first category, the devices rely on measuring frequency shifts. However, all such devices require extremely accurate and precise components which are either unavailable or are too expensive to be practical. Furthermore, such devices lack stability in operation.
In the second category are devics which measure variable pulse widths. However, these devices also suffer serious stability drawbacks and in any event use active components which inherently result in complex circuits and usually require a power supply.
By contrast, the present system and method utilizes an entirely passive sensor circuit which is first attached to an object and which is intermittently energized or rung at its resonant frequency by an oscillating signal burst from a remote indicator unit. The sensor circuit temporarily stores the energizing energy and then reradiates that energy. The rate of decay of the reradiated ring signal after each signal burst varies depending on the Q of the sensor circuit which is dependent, for example, upon the resistance of the thermistor and hence the temperature of the body with which the thermistor is in contact. The rate of decay of the ring signal is sensed and processed by the remote indicator unit to obtain a value of Q and therefrom the parameter value, such as temperature, which is then displayed.
Thus, the present invention does not suffer from the instabilities and complexity of prior art devices.